Instead of such a mass of _dead_ protein, however, take a particle of _living_ protein--one of those minute microscopic living things which throng our pools, and are known as Infusoria--such a creature, for instance, as an Euglena, and place it in our vessel of water. It is a round mass provided with a long filament, and except in this peculiarity of shape, presents no appreciable physical or chemical difference whereby it might be distinguished from the particle of dead protein. But the difference in the phaenomena to which it will give rise is immense: in the first place it will develop a vast quantity of physical force--cleaving the water in all directions with considerable rapidity by means of the vibrations of the long filament or cilium. Nor is the amount of chemical energy which the little creature possesses less striking. It is a perfect laboratory in itself, and it will act and react upon the water and the matters contained therein; converting them into new compounds resembling its own substance, and at the same time giving up portions of its own substance which have become effete. Furthermore, the Euglena will increase in size; but this increase is by no means unlimited, as the increase of a crystal might be. After it has grown to a certain extent it divides, and each portion assumes the form of the original, and proceeds to repeat the process of growth and division. Nor is this all. For after a series of such divisions and subdivisions, these minute points assume a totally new form, lose their long tails--round themselves, and secrete a sort of envelope or box, in which they remain shut up for a time, eventually to resume, directly or indirectly, their primitive mode of existence. Now, so far as we know, there is no natural limit to the existence of the Euglena, or of any other living germ. A living species once launched into existence tends to live for ever. Consider how widely different this living particle is from the dead atoms with which the physicist and chemist have to do! The particle of gold falls to the bottom and rests--the particle of dead protein decomposes and disappears--it also rests: but the _living_ protein mass neither tends to exhaustion of its forces nor to any permanency of form, but is essentially distinguished as a disturber of equilibrium so far as force is concerned,--as undergoing continual metamorphosis and change, in point of form. Tendency to equilibrium of force and to permanency of form, then, are the characters of that portion of the universe which does not live--the domain of the chemist and physicist. Tendency to disturb existing equilibrium--to take on forms which succeed one another in definite cycles--is the character of the living world. What is the cause of this wonderful difference between the dead particle and the living particle of matter appearing in other respects identical? that difference to which we give the name of Life?